At appropriate times, as when refueling of the nuclear reactor is to be done or repairs are to be made within any of the water circulating loops, the level of the primary coolant water within the system must be lowered usually to a level below that of the horizontally disposed inlet and outlet pipes extending to and from the reactor. In the usual reactor system which serves several vertical U-tube type steam generators, the draining down is usually effected through both the chemical volume control system and the residual heat removal system, from which the draining water is led into holdup tanks. As is well known, such draining down is conducted only after the system has been cooled and depressurized by releasing steam from the steam generators, shutting down the main circulating coolant water pumps, depressurizing the pressurizer, and starting the residual heat removal pumps to actuate the residual heat removal system. The draining down procedure is initially monitored using the pressurizer water level indicator until all of the water has been drained from the pressurizer, after which it is monitored using the low-level water monitoring system and by measuring the amount of water received in the holdup tanks, until completion of the drain down.
As is also well known, during the drain down a gas which is inert to the system, such as nitrogen is introduced into the system through the top of the system pressurizer tank, to displace the water which is draining from the system. When the water is drained from the pressurizer, whose discharge flows into one of the "hot legs", or reactor outlet conduits, which leads to the bottom of one of the steam generators, the nitrogen introduced through the pressurizer will begin to disperse throughout system from the bottom of the pressurizer. The nitrogen first enters the reactor above the water level therein via the reactor outlet conduit to which the pressurizer is connected, and then passes via all of the reactor inlet and outlet conduits, which are horizontally disposed at the same elevation, to the respective steam generators and main circulating pumps in the respective steam generation loops. However, the inverted U-tube bundles in the respective steam generators, which extend upwardly from the elevation, drain slowly and sporadically due to vacuum formation at the tops of the inverted U-tubes, and such unpredictable flow is a familiar phenomenon to those skilled in the art.
This sporadic or intermittent flow-down of the water from within the inverted U-tubes is due to the interruption of the flow of the nitrogen into the U-tubes from the reactor inlet and outlet conduits which is caused by surges of water from the U-tubes themselves. That is, in much the same way as the water in a suddenly inverted bottle will be sporadically discharged from its downwardly facing neck only as fast as air bubbling in the opposite direction relieves the vacuum which constantly forms at the upper, closed end of the bottle, the water which is sporadically surging in considerable quantities from the tube bundles spills back into the reactor inlet and outlet conduits, closing off the flow of nitrogen therethrough, whereupon additional water cannot drain from the tube bundles because of the vacuums forming at their inverted, upper ends. Moreover, these surges create back pressures in the system which force quantities of the nitrogen to become entrapped, for example within the seal packages of the circulating pumps, so that nitrogen circulation is interrupted. In this regard, it will be understood that after nitrogen emerges from the underside of the pressurizer its pressure has been reduced virtually to atmospheric pressure and that the nitrogen is then in an expanded condition, so that it is unlikely to continue to flow into the steam generator U-tubes against the weight of the water draining from the tubes.
Thus, the familiar "gurgling" of the drain-down water occurs, causing intermittent significant upward readings in the system level indicators, and significant time intervals, sometimes of several hours duration, before the draining down, which is self-controlled by the system, continues. Thus, for example, in a typical Westinghouse nuclear powered system having four steam generators, the time required to drain down the primary coolant water from the system to the level at which the steam generators have been emptied averages about 32 hours. Where the draining down is only for the purpose of checking the tubes in the steam generators or for making repairs within the primary coolant system itself, this time duration poses a significant limitation upon the early completion of such inspection or repairs.
Accordingly, it is intended by the present invention to provide a method and means for significantly reducing the time required for draining down the primary coolant water in such a system. Moreover, it is intended by the present invention to make the drain-down process a continuous one, with no delays being incurred due to sporadic draining of the water from the generator U-tubes.
Because the taking of direct visual readings on the reactor cooling system low-level monitors is believed necessary during the drain-down procedure, it is a further object of the present invention to significantly reduce the number of such direct readings as are required during the drain-down operation, to thereby significantly reduce the exposure of the operator to radiation, as occurs whenever such direct readings are taken. That is, using conventional drain-down techniques, standard procedure requires the drain-down operator to enter the vapor containment area every hour to read the reactor coolant system level, each trip resulting in approximately 10 milirems of exposure to radiation. By the present invention it is intended to increase the reliability and confidence of the operator in the indirect level monitoring system, and thereby reduce the frequency of such direct readings taken within the containment. For example, using the present invention, such direct readings need be taken only once per eight hour shift, as compared to hourly, as aforesaid.
It is also known that such accurate and careful monitoring of the level of the draining reactor coolant water is necessary for maintaining a positive suction head on the residual heat removal pumps to prevent cavitation in the pumps and consequent pump damage. Accordingly, it is intended by the present invention to promote an accurate and continuous drain-down, so as to assure that such positive suction head is always maintained on the residual heat removal pumps, as a matter of course.